Among various plastics, fluorine-containing resins represented by polytetrafluoroethylene (hereinafter referred to as PTFE) and tetrafluoroethylene-hexafluoropropylene copolymer (hereinafter referred to as FEP) have particularly excellent heat resistance, chemical resistance, electrically insulating property, non-tackifying property, low friction property and the like, and thus have been widely used in the fields of not only chemical industry, electric and electronic industries and machine industry but also space development, aircraft industry and household goods. In order to obtain those fluorine-containing resins, any of polymerization processes such as bulk polymerization, solution polymerization, suspension polymerization and emulsion polymerization can be used. However, in bulk polymerization, it is difficult to regulate polymerization temperature because of large reaction heat generation, and it is difficult to take a produced polymer out of a reactor. In solution polymerization, a large amount of organic solvent is required and recovery thereof is necessary. Thus those polymerization processes are not suitable for industrial large-scale production. Usually suspension polymerization and emulsion polymerization have been used since regulation of polymerization temperature is easy and taking out a produced polymer, washing and solid/liquid separation are relatively easy. A feature of those two polymerization processes is the use of water as a reaction medium.
A fluorine-containing olefin monomer which is in gaseous phase at normal temperature generally has a very low solubility in water, which is a factor of making it impossible to sufficiently increase polymerization rate. In order to increase a monomer concentration in polymerization system (liquid phase), addition of a dispersant and emulsifing agent and increase in polymerization pressure have been conducted. Thereby though the polymerization rate can be increased to a certain extent, the polymerization rate-determining step would be not the polymerization reaction (growth reaction) itself but still the diffusion of monomer into the aqueous medium. It is if the polymerization reaction can become the polymerization rate-determining step, the highest polymerization rate which can be considered theoretically can be achieved. In conventional polymerization procedures, since diffusion of monomer was the polymerization rate-determining step, only the polymerization rate of at most up to 75% of the theoretical polymerization rate could be obtained. This is because, in conventional polymerization procedures, gas/liquid contact efficiency is not sufficient as compared with very high polymerization reactivity of fluorine-containing olefin monomers, thus monomer supply being unable to catch up with the high polymerization rate.
The reason why diffusion of monomer is the polymerization rate-determining step is observed in the fact that in case where polymerization conditions other than conditions for stirring system are the same, polymerization rate varies by changing a stirring speed and shapes of stirring blades and a reaction tank. In case where the polymerization reaction itself is the polymerization rate-determining step, the polymerization rate should not vary even if the stirring system is changed. Hitherto there have been no reports that when a fluorine-containing olefin monomer was polymerized in an aqueous medium, the polymerization reaction (growth reaction) but not the diffusion of monomer was employed as the polymerization rate-determining step. On the contrary, as described in examples of U.S. Pat. No. 5,266,639 and JP-B-61-43364, in order to keep a polymerization pressure and polymerization rate constant during the polymerization reaction, rotation speed for stirring has been changed properly.
An object of the present invention is to achieve a higher polymerization rate, that is, a higher polymerization productivity than that of conventional polymerization process by employing, as the polymerization rate-determining step, the polymerization reaction (growth reaction) itself or the reaction close thereto but not the diffusion of monomer when monomers containing fluorine-containing olefms are subjected to batch polymerization in an aqueous medium.